1989 Ninety fuel sender/gauge wiring

The fuel gauge in my son's Ninety has just recently decided to stop reading correctly, actually doesn't read at all now. It has been a bit erratic past few weeks so we thought probably the sender unit playing up.
Slate grey wire on sender has 12 volts, low level light I think.
Black earth tested for continuity, ok.
Green/black trace when connected to test light (test light only glows) the gauge starts moving. When tested with multimeter its only showing 7 volts, would I be correct in thinking it should have the full 12volts on this wire?
Many thanks.

At one point mine failed the sending unit in the tank a replacement fixed the issue.
Otherwise wiring & the gauge, the ground.

Thanks for the reply. I suspect it is the sender but wanted to know if wiring maybe at fault first, gauge seems OK.
Incidedently what make did you buy, is it worth getting a Britpart special?

If your gauge is moving when green black is grounded then problem is with sender. Getting a replacement to read correctly can be fun.
With mine it was a case of looking in the tank and judging level [ having half is good ] and then bending the wire arm a bit so when held in place gauge reads right.

I have been running a britpart sender for a few years with no issue. I would also agree with the above that the accuracy is questionable But I think that is just a land rover issue as my td5 has the same issue. The guage is just a guide and you are much better resetting the trip meter every fill up and judging it on mileage rather than what the gauge says.

Tested the wire at the gauge and it looks like 7volt is correct, so looks like its time for a new sender unit.
Thanks for the replies.

Bought a new budget sender unit, Bearmach arrived, and it seems to work.
Fitting not exactly straight forward as the original is metal, this one is plastic, so the tabs on the retaining ring needed to be bent a little.